The present invention relates to a positioning device, and more specifically, to a positioning device used in an automatic changing apparatus of an optical memory system.
In general, an automatic disk changing apparatus of an optical disk system serves to take out a desired optical disk cassette, from amongst a number of other cassettes arranged in layers, by use of transfer means, and to load the selected cassette into a disk drive system. Also, the transfer means is used to return an optical disk cassette in the drive system to its original position. For example, an AC servomotor is used as a drive source for the transfer means.
Conventionally, a positioning device for positioning the transfer means has a rotary encoder, which rotates in association with the movement of the transfer means. The encoder produces position feedback pulses which are proportional to its rotational angle. When command pulses, which correspond to the distance in which the transfer means is to be moved, are applied to command-pulse input means, the pulses are added to a deviation counter through a pulse-shaping and rotating-direction-control circuit. The pulses integrated by the deviation counter are converted into a speed command voltage by a D/A converter. A servo driver uses this command voltage to drive the AC servomotor.
As the transfer means is moved by means of the motor, the rotary encoder rotates, and delivers position feedback pulses which are proportional to the rotational angle of the encoder. These pulses are applied to the deviation counter via the pulse-shaping and rotating-direction-control circuit. The feedback pulses are subtracted from the command pulses which are applied previously to the counter. As a result, the motor is rotated through an angle corresponding to the result of the subtraction, thereby moving the transfer means to a specified position.
At the start of rotation of the motor, only the command pulses are applied to the deviation counter, so that the motor is accelerated. Thereafter, the frequency of the feedback pulses agrees with that of the command pulses, so that the motor rotates at constant speed. In the end, the command pulses cease to be applied to the counter, and only the feedback pulses continue to be input thereto. Thus, the motor is decelerated.
In the positioning device constructed in this manner, the current position of the transfer means is unknown at the time when power is turned on. Therefore, the device is provided with a sensor for detecting the transfer means being situated in a desired position, next to the rotary encoder. When the power is turned on, the transfer means is moved to a position where it faces the sensor, and this position is detected as the current position of the transfer means. In moving the transfer means to the desired position, the difference between the current position and the desired position is calculated. The input means is supplied with command pulses, which, based on the calculated value, correspond to the distance the transfer means is to be moved.
According to the prior art positioning device, as described above, the transfer means must be moved temporarily to a predetermined position when the power is turned on, thus requiring much starting time. Also, such a device requires a sensor or other position-detecting means, which entails an increase in the number of components. Moreover, the movement of the transfer means must be controlled in consideration of detection data from the sensor, that is, the control of the transfer means is complicated.